In mobile communications schemes, transmit power and communications (transmit) rate is controlled in accordance with the type of communications data such as audio communications and packet communications for realizing desired quality thereof.
Wideband-Code Division Multiple Access (W-CDMA) controls transmit power in such a manner that a Signal to Interference Ratio (SIR) is maintained constant (hereinafter will be called the previous example 1). Further, control called outer loop for obtaining the SIR corresponding to a desired BLER is also performed since signal quality [an error rate such as a Block Error Rate (BLER)] and the SIR depend on propagation circumstances.
This control method performs control such that a instantaneous SIR is made to be constant with respect to the time unit of measuring a BLER for realizing a desired BLER. Thus, transmit power is enlarged under a condition where propagation circumstances are poor. However, there are cases where a total transmit power can be decreased by deteriorating quality when propagation circumstances are poor and improving the quality when circumstances are fine.
Accordingly, a control method which finds a relationship such that an error rate (BLER) and a reception condition (reception feature) becomes 1 to 1, being independent from propagation circumstances, is proposed in, for example, the following patent document 1 (hereinafter will be called the previous example 2).
Normally, a relationship of BLER relating to a Signal to Noise Ratio (SNR) or a power density to noise power density ratio Eb/N0 or the like depends on propagation circumstances as shown in FIG. 19, so that it is impossible to give a 1-to-1 relationship. In this instance, this FIG. 19 is on the assumption that the number of information bits (transmit block size) for a single code block (1 coding unit) is 3000. In the figure, the curve indicated by the reference character 100 is an error rate (BLER) under static circumstances with no fading; the curve indicated by the reference character 200 is an error rate (BLER) under infinite circumstances in which the speed of fading is extremely high; the curve indicated by the reference character 300 is an error rate (BLER) under circumstances in which a fading frequency is about 3 Hz; the curve indicated by the reference character 400 is an error rate (BLER) under circumstances in which a fading frequency is about 80 Hz; the curve indicated by the reference character 500 is an error rate (BLER) under circumstances in which a fading frequency is about 240 Hz.
In a system employing a strong error correction codes such as turbo codes or LDPC codes, it is conceivable that a tendency of capacity being close to the theoretical value thereof will be exhibited. In a case where transmit power is controlled in such a manner that an evaluation amount C(P) (hereinafter also simply called the “evaluation value C”) expressed by the following formula (1) maintains constant by the unit of code block, an error rate relating to an evaluation amount C has approximately the same feature (1-to-1 relationship) independent from propagation circumstances such as fading as shown in, for example, FIG. 20. In this instance, the curves (features) indicted by the reference characters 100, 200, 300, 400, and 500 in this FIG. 20 correspond to those shown in FIG. 19.
                              C          ⁡                      (            P            )                          =                              1            M                    ⁢                                    ∑                              i                =                0                                            M                -                1                                      ⁢                          log              ⁡                              (                                  1                  +                                      α                    ⁢                                                                                  ⁢                                                                                            A                          i                                                ⁢                                                  P                          i                                                                                            N                        i                                                                                            )                                                                        (        1        )            where P is transmit power; A is a channel value in propagation circumstances such as fading; N is interference and noise power; α is a constant; M is the number of symbols or the number of blocks each formed by more than one symbol. FIG. 20 shows a case where turbo codes with a coding rate of ⅓ and a constraint length of 4 is used, and α=2 in the above formula (1). Hereinafter, the evaluation amount C in the above formula (1) will be called AC (Approximated Capacity).
From this relationship between an error rate (BLER) and an AC, being independent from propagation circumstances, the technique of the previous example 2 performs transmit power control in such a manner that an AC maintains constant, and performs transmit power control following the water filling theory in the unit of code block (coding), thereby obtaining a desired BLER with a transmit power as small as possible. In this case, performing control in such a manner that an AC maintains constant means that control is performed indirectly in such a manner that the BLER of each code block maintains constant.
Patent Document 1: Japanese Patent Application Laid-open No. 2005-12321